Treatment of tannin extracts



United States Patent eral Foods Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application December 22, 1949,

Serial No. 134,583

11 Claims. (Cl. 260-4735) The present invention relates to vegetable tannin extracts, and more particularly to a method whereby one may upgrade such extracts or increase the tannin content thereof.

The last few decades have seen considerable improvement in connection with leather tanning processes. Much of such improvement depends upon the use of relatively high grade tannin extracts, i. e., those having a relatively high tannin content, such as quebracho wood, mangrove bark and chestnut wood extracts. Of these materials only the latter is available domestically. The extracts of the other domestically available materials such as pine bark, hemlock bark, canaigre root, redwood and the like are of relatively low grade and, accordingly, are not generally suitable. Thus, the bulk of the vegetable tannin materials employed in this country must be imported. Moreover, the situation is aggravated by the fact that the chestnut tree is being rapidly exterminated by a fungus called chestnut blight for which no eflective control has yet been found.

The problem of obtaining suitable tanning materials from domestic sources could be dealt with by: (1) developing chemical tanning agents (2) the development of new domestic sources of vegetable tannin materials and (3) upgrading or increasing the tannin content of the domestically available vegetable tannin materials. That the first solution to the problem has been extensively investigated is well evidenced by the numerous chemical tanning agents that have been developed in recent years and the extent to which they have been used in the industry. Examples of such tanning agents are the sulphated polyphenols and the chromium salts, but numerous other organic and inorganic agents have also been found effective. That considerable efiort has been made to find new domestically available sources of vegetable tannins is seen in the development and use of canaigre root extracts and certain by-products from the lumbering and paper industries such as lignin sulphonic acid. Thus far, however, no discernible resort has been had to the third of the above-proposed solutions to the problem.

An object of the present invention is to provide a process whereby vegetable tannin extracts may be upgraded in a practical and economical manner.

Another object of the present invention is to provide vegetable tannin extracts of a higher tannin content than has ordinarily been available heretofore.

Still another object of the invention is to provide from relatively low grade vegetable tannin extracts, tannin extracts of sufficiently high grade to be useful in the economical tanning of leather.

A further object of the invention is to provide extracts of higher tannin content from particular vegetable tan nin source materials.

A still further object is to provide a process whereby tannin extracts from domestic sources may be rendered suitable for use in the tanning industry.

- base.

2,753,371 Patented July 3, 1956 "ice Other objects will be apparent from the descriptive matter following hereinafter.

It is now proposed to upgrade vegetable tannin extracts or increase the tannin content thereof by a process which involves reacting in aqueous media the tannins of such extracts with certain nitrogenous bases, to be described hereinafter, to form a substantially water-insoluble complex which is separated and then treated with a water-immiscible, organic solvent for the nitrogenous Such treatment results in the resolution of the complex with the separation of the tannins and the nitrogenous base. The aforementioned base is then recovered from the organic solvent solution and may be used in the upgrading of a subsequently treated tannin extract. The tannin material remaining after removal of the base is of increased tannin content and is more effective for use in leather tanning.

The nitrogenous bases which may be employed in the process of the present invention are those which are heterocyclic, contain a tertiary nitrogen atom, have a basic dissociation constant less than 1 1O and are at least moderately soluble in an aqueous liquid and in a water-immiscible organic solvent. It is not necessary that the atoms forming the ring structure be limited to nitrogen and carbon, but instead sulphur, oxygen and perhaps other atoms may also be present in the ring. This is shown by the fact that nitrogenous bases such as the various thiazoles, oxazoles and isoxazoles may be employed in the process. As is generally understood, the designation of one of the nitrogenous atoms of the base as tertiary indicates a nitrogen atom to which no hydrogen atom is attached.

Examples of the hereinbefore defined nitrogenous bases which have been found useful in preparing the insoluble tannin complex are:

l-phenyl-Z,3-dimethylpyrazolone-5 1-phenyl-2-ethyl-3-methylpyrazolone-5 l-o-tolyl-2,3-dimethylpyrazolone-5 1-o-tolyl-2-ethyl-3-methylpyrazolone-5 V 1,Z-di-m-tolyl-3-methylpyrazolone-5 1-p-tolyl-2,3-diInethylpyrazolone-5 1-p-tolyl-2-ethyl-3 -methylpyrazolone-5 1-phenyl-3 -ethyl-4-methylpyrazolone-5 1-phenyl-3-rnethyl-4-propylpyrazolone-5 1-phenyl-2-butyl-3methylpyrazolone5 4-methylthiazole 2,4-dirnethylthiazole 2,5-dimethylthiazole 2-ethyl-4-methylthiazole 2-methy1-4-ethylthiazole 2,4,5-trimethylthiazole 2,4-dimethyloxazole 2,5-dimethyloxazole 2,4,5 -trimethyloxazole 2,5-dimethyl-4-isopropyloxazole 2,5 -dimethyl-4-isobutyloxazole 3,5 -dimethylis0xazole 3-methyl-5-propylisoxazole 3,4,5 -trimethylisoxazole l-ethylpyridone-Z 1,2,4-trimethylpyridone-6 l-methylpyridone-2 1-methylpyrro1idone-5 l-ethylpyrrolidone-S l-phenylpyrrolidone-S 1,2-dimethylpyrrolidone-5 2,3-dimethylpyrazine 2,5 -dimethylpyrazine 2,3,5-trimethylpyrazine 2-methyl-3-propylpyrazine 2,5-dimethyl-3-ethylpyrazine 3 2,5-dimethylpyrazine Tetramethylpyrazine 2,5-diisopropylpyrazine :2g5-"dimethyl-3,Gdiethylpyrazine Caffein Vegetable tannins which may 'be' obtained by the aqueous extraction of the different source materials such as variousbarks, woods, roots, etc., have been divided into two groups in 'a system of classification devised by Freudenberg. 'l hose' tannins which can be hydrolyzed by. a'cids-aznd enzymes are designated as hydrolyzable tannins. while those which cannot be so hydrolyzed are referred. to as condensed tannins. Various vegetable tannin extracts contain hydrolyzable or condensed tannins, exclusively, while others are composed of a mixture of hydrolyzable and condensed tannins in varying proportions. The vegetable tannins contained in the extracts obtained from che'stnut wood and myrobalans nuts are substantially all'of the hydrolyzable type. On the other hand, the extracts obtained from quebracho wood, mangrove bark, wattle bark, hemlock bark, and gambier leaves and twigs contain tannins which are substantially all of the condensed type. Extracts such as those of valo'nia cups and beards and chestnut oak bark contain both types of tannins. While the present invention is applicable to both of the aforementioned types of vegetable tannins and to all types of extracts thereof, it has been found to be particularly effective when applied to extracts. consisting of or containing mainly condensed tannins and for thisireason is preferably applied to such extracts.

The reaction between the vegetable tannins and the aforementioned nitrogenous bases to form the insoluble complex is carried out in aqueous media. This is generally accomplished by dissolving and/or suspending the tannin in an aqueous medium andadding the nitrogenous base thereto. If desired, the base may be added in solid form although it is preferred to add it dissolved and/or suspended in an aqueous or water-miscible medium. While it is possible to form the complex by adding the tannin soluti'onor suspension to a solution "or suspension of the base, it has been foundthat the tannin-base complex is somewhafsolubl'e in the "presence of excessively high concentrations "of base-and for this reason this procedure is not preferred. This is not intended, however, to indicatethat the manner "in which 'the' complex may be formedi's restrictedto that first'g'iven'. Rather it merely indicates that the first-rhethodprovide's a greater degree of control ofther'at'e ofp'recipitation. This in turn provides for less occlusion, co precipitation, etc., which must always be minimized in-carryin' -eut "any purification or separation by means of a'proce'ssdnvolving precipitation.

Because the tannins contained in 'the'ta'nnin extracts which are processed in accordance with the present invention are of unknown identity and considerable complexity it is impossible to statewith anyd'egree of certainty the proportion of a particularnit'rogenous base which should be employed-i'n treating the various tannin extracts. It has been foun'd, however, that nitrogenous bases having molecular weights-oftheorderof those listed above may be employed with satisfactory results in an amount equal to the tannincont'entof th'e'tannin extract. In the case of bases of appreciably loweror higher molecular weights the proportions that should be used are accordingly decreased and increased, respectively. In any event, the base or the solution thereof is usually added to the solution and/or suspension-of the tannin extract until it becomes apparenttha-t the further addition of base does not result in oompl'ex' formation. Usually the amount of base will be from about two-tenths to about four times the weight of the tannins contained in the extract to be upgraded.

The bases with which the-process of' the present invention is concerned arev subs'tan'tially 'water soluble.

While it may be necessary that the base be dissolved to some extent in order for the reaction involved in the formation of the complex to occur, it is not necessary that a high degree of solubilization be had or that all of the base employed be in solution at the same time. It is suitable to employ a suspension of undissolved base in the reaction mixture since such does not interfere with the purification or occasion a loss of base as will be seen hereinafter. It is also suitable for the purposes of the invention to solubilize the base to some extent, as, for example, by the use of a small amount of acetic acid which serves to form the acetate of the base which in most cases is more soluble than the base itself. Also, it may be desirable in some cases to employ a small proportion of such volatile, water-miscible co-solvents as acetone, dioxane, methanol, ethanol and the like.

The employment of such solvents as those mentioned immediately above shou-ldbe minimized, however, since this involves extra expense and such solvent sa-re usually extracted by the, water-immiscible solvent for the base which is used in recovering the excess thereof from the liquid from which the complex has been separated as described hereinafter. The continued use of such cosolvents could-thus result in the accumulation of relatively large amounts thereof in the aforementioned water-immiscible solvent for the base and ultimately require a fractionation to effect their separation. In view of the latter considerations it is desirable to employ the more volatile of the aforementioned co-solvents if the need for such is regarded as being sufliciently great to warrant their use, at least when using the chlorinated hydrocarbon solvents which are those generally preferred as is pointed out hereinafter.

While the complex formation may be carried out over a wide range of temperatures, temperatures as high as 50 C. being suitable, it is preferred to carry out said formation at temperatures below room temperature, still more preferably at temperatures below 15 C. The use of low temperatures insures obtaining the maximum amount of complex and also provides one more easily handled in the subsequent separation and washing steps. The obvious lower limit of such temperatures is the freezing temperature of the aqueous medium in which the complex formation iscarried out.

The means used for separation of the complex depends primarily upon the physical character of the complex which may range from a fairly hard solid to an oil. Here again the use of a low temperature, say between the freezing. temperature of the supernatant liquid and 15 C., is preferable since at such temperatures the complex tends to :solidify and. in any event is more easily handled. If the complex is in the form of an oil or a soft solid it 'is usually desirable to effect the separation by centrifugation although sedimentation or gravity settling and decantation may also be employed. When suitably hard, the complex may be separated by filtration.

After the complex has been separated it is preferred that it be washed to remove adhering non-tannin material. This is best accomplished using Water or other aqueous "liquids, preferably cold, in which the non-tannin materialsaresoluble and the complex substantially insoluble. These washings are saved for subsequent extraction with a water-immiscible solvent for the nitrogenous base and are preferably combined with the aforementioned supernatant liquid or filtrand for the purpose of extracting the base. The solvent used for thispurpose is preferably the same as thatused for the'resolution of the complex discussed hereinafter.

The separated complex is. broken up or resolved by extractio-nof the-nitrogenousbase therefrom with a waterimmiscible organic solvent for said base. This may be carried out by leaching if the complex is a sufficiently hard s'oli'd'to'be adaptable to such a means of extraction. The solid complex i'sfsubdivided'u'sually by grinding, and then subjectedto theaction'of the solvent. After the extraction has continued for a period of time it is usually advisable to subdivide or grind the solid again in order to present new surfaces to the leaching solvent and thus aid the extraction. If the complex is an oil it may be similarly directly treated in the liquid-liquid manner with a solvent for the nitrogenous base. It is better, however, that the oil be suspended in water or other aqueous media for such liquid-liquid treatment. Even in those cases where the complex is in the form of a soft or relatively hard solid it is preferred that it be subdivided to whatever extent is practical and suspended in water or other aqueous liquids for treatment in the liquid-liquid manner with the solvent for the base. This manner of proceeding has numerous advantages not only from the standpoint of providing greater ease of handling but also from the standpoint of decreasing the amount of time required and increasing the efficiency of the resolution.

The water-immiscible solvents which may be used for the resolution of the tannin-nitrogenous base complex are those water-immiscible organic solvents for the nitrogenous base such as C2HC13, C2Cl4, CCl4, CHClz, C2H4Cl2, C2H2Cl4, benzene, toluene, hexane, heptane, the low boiling petroleum ethers, ethyl ether and the like. While in general ethyl ether is superior with respect to its solubilizing effect on the nitrogenous bases, practical operating considerations based on its extreme volatility and inflammability make its use less desirable than that of some of the other organic solvents. In most cases it is preferred to use the chlorinated hydocarbon solvents inasmuch as they generally have a suitably high solvent effect, are

non-inflammable and less volatile. Hydrocarbons such as hexane and benzene are the least desirable solvents because of their low solubilizing effect on the nitrogenous bases, their high volatility and their inflammability.

Generally, the particular base used in the process of the invention should be selected not only on the basis of cost, but also with the view to the efiiciency with which it combines with the tannins of the particular extract being processed, the character of the complex which it forms and the convenience with which it may be recovered from the separated complex by extraction with a water-immiscible solvent therefor such as those aforementioned. For example, other things being equal, a base which could only be recovered from the complex within a practical amount of time by using a highly inflammable or highly volatile solvent would not be preferred to a base which could be readily extracted with a less volatile, non-inflammable solvent.

The distribution coefficient of the nitrogenous base with regard to the water-immiscible solvent and the aqueous medium involved when the complex resolution is effected by liquid-liquid techniques must be taken into consideration. It is desirable that the nitrogenous base be preferentially soluble in the former since this permits the volume of said solvent to be small. However, when there is only a small preferential solubility, and even when the base is preferentially soluble in the aqueous medium, complete resolution of the complex and transfer of the base from the aqueous medium to the organic solvent may, nevertheless, be effected. If such is the case, however, larger volumes of the organic solvent are required for such resolution and transfer and it is usually preferable not to use liquid-liquid extraction or at least to avoid the use of aqueous media.

In light of the above considerations one of the preferred bases to be employed in accordance with the practice of the present invention is caffein. This base is not only available at reasonable cost but, also, is readily soluble in water and readily reacts with the tannins to form the complex. Further, it is readily recovered from the separated complex by extraction with the commonly available chlorinated hydrocarbons such as C2014, CC14, CHCls, C2HC13 and the like. Of these various solvents, CzHCls is preferred from the standpoint of its greater availability,

lower cost and excellent solubilizing ability with respect to caflein.

In view of the relative expense of most of the nitrogenous bases used in forming the complex it is desirable to recover as much of the same as possible. Accordingly, the aqueous medium in which the complex was formed and from which it was separated, i. e., the filtrand, supernatant liquid, etc., is extracted with a water-immiscible solvent for the base. This serves to recover any excess of the reagent that may have been employed. it is also advisable to treat similarly the washings from the complex, that is, the water or other aqueous liquid used in washing uncombined materials from the separated complex. It is preferred from the standpoint of convenience that the supernatant liquid, filtrand or the like be combined with said washings and treated to remove the base at the same time.

The recovery of the nitrogenous base contained in the water-immiscible organic solvent solution resulting from the resolution of the tannin-base complex and that resulting from the extractionof the washings and the supernatant liquid, filtrand or the like mentioned immediately above, is best obtained by distillation of the solvent. It is preferred that the solvent used in recovering the base from the various aqueous liquids be the same as that used in the resolution since this simplifies its distillation. Also, the use of relatively low boiling solevents is preferred since it reduces the cost required for distillation. In some cases, it is satisfactory to recover the base from the organic solvent by liquid-liquid extraction with water, especially if the distribution coefficient favors its solubility in water. Small amounts of water-miscible co-solvents for the base can also be e1nployed in the aqueous extraction when they increase the preferential solubility of the base in the aqueous liquid. Another device that can be resorted to for the same purpose is to elevate or lower the temperature of the extraction. Extraction with water for recovery of the base has the advantage of providing the base already in solution for use in the treatment of a subsequently processed tannin extract.

Generally, the base recovered by distillation of such solvent is in a somewhat impure condition, at least as compared with the material that may have been initially introduced into the process. The impurities can usually be removed from the base by forming a concentrated solution in fresh water-immiscible solvent and thereafter clarifying the same. The polymerized materials constituting the greater amount of said impurities are not soluble in most fresh organic solvents.

The upgraded tannin material resulting from the resolution of the aforementioned complex may be recovered in dry form if desired or in the form of a concentrated solution by distillation, evaporation and the like of the water or aqueous liquid. It is generally preferable to reduce the tannin extract down to a concentrated solution of thick, syrupy consistency instead of to a dry powder because in doing the latter some water-insoluble materials are formed during drying, probably by polymerization, which are undesirable when the dry extract is reconstituted for tanning purposes.

The following specific examples illustrate the results obtained by the application of processes embodying the present invention to tannin extracts derived from the various sources indicated.

Example I A 400 g. sample of dried canaigre root extract containing 46% tannins was dissolved in 3 l. of water and cooled to nearly 0 C. in an ice bath employing suitable agitation. A hot solution comprising 200 g. of catfein dissolved in 1 l. of water was slowly added with simultaneous stirring. There formed during this operation a fine, granular tannin-caffein complex. During the complex formation the temperature of the reaction mixture 9 more than substantially 1X10 and containing at least one tertiary nitrogen atom, separating the thus formed complex and extracting said base from said complex with a water-immiscible organic solvent for said nitrogenous base.

3. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with caffein, separating the thus formed complex and extracting said caffein from said complex with a waterimmiscible organic solvent for cafiein.

4. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting at a temperature within the range between the freezing temperature of the aqueous medium employed and about 15 C. the vegetable tannin content thereof with a water-soluble heterocyclic nitrogenous base having a basic dissociation constant of not more than substantially l 10- and containing at least one tertiary nitrogen atom, separating the thus formed complex and extracting said base from said complex with a waterimmiscible organic solvent for said nitrogenous base.

5. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with a water-soluble heterocyclic nitrogenous base having a basic dissociation constant of not more than substantially 1 10 and containing at least one tertiary nitrogen atom at a temperature within the range between the freezing temperature of the aqueous medium employed and about 15 C., separating the thus formed complex at a temperature within said range of temperatures and extracting said base from said complex with a waterimmiscible organic solvent for said nitrogenous base.

6. A process of upgrading a vegetable tannin extract the tannin content of which consists substantially of condensed tannins which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with caifein, separating the thus formed complex and extracting said caffein from such complex with a water-immiscible organic solvent for caffein.

7. A process of upgrading a Vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with caffein, separating the thus formed complex, extracting said caffein from said complex with a water- 10 immiscible chlorinated organic solvent for caffein and recovering the thus separated vegetable tannins.

8. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with caffein, separating the thus formed complex, extracting said cafiein from said complex with trichlorethylene and recovering the thus separated vegetable tannins.

9. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with catfein at a temperature within the range between the freezing temperature of the aqueous medium employed and about 15 C., separating the thus formed complex at a temperature within said range of temperatures, extracting said caffein from said complex with trichlorethylene and recovering the thus separated vegetable tannins.

10. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with caftein, separating the thus formed complex, distributing the separated complex in an aqueous medium, liquidliquid extracting said catfein from said complex with trichlorethylene and recovering the thus separated vegetable tannins.

11. A process of upgrading a vegetable tannin extract which comprises forming a substantially water-insoluble complex by reacting the vegetable tannin content thereof with cafiein at a temperature within the range between the freezing temperature of the aqueous medium employed and about 15 C., separating the thus formed complex at a temperature within said range of temperatures, distributing the separated complex in an equeous medium, liquid-liquid extracting said caflein from said complex with trichlorethylene and recovering the thus separated vegetable tannins.

Faunce Dec. 30, 1902' Batchelder Mar. 28, 1939 OTHER REFERENCES Analyst 50, pp. 162-6 (1925). 

1. A PROCESS OF UPGRADING A VEGATABLE TANNIN EXTRACT WHICH COMPRISES FORMING A SUBSTANTIALLY WATER-INSOLUBLE COMPLEX BY REACTING THE VEGETABLE TANNIN CONTENT THEREOF WITH A WATER-SOLUBLE HETEROCYCLIC NITROGENOUS BASE HAVING A BASIC DISSOCIATION CONSTANT OF NOT MORE THAN SUBSTANTIALLY 1X10-12 AND CONTAINING AT LEAST ONE TERTIARY NITROGEN ATOM, SEPARATING THE THUS FORMED COMPLEX AND EXTRACTING SAID FROM SAID COMPLEX WITH A WATER-IMMISCIBLE ORGANIC FOR SAID NITROGENOUS BASE. 